The present invention relates to digital image forming apparatuses, such as digital copying machines for copying a large or long original document, in which an image of the original document is formed by reading the image with a one-dimensional image sensor such as a CCD and writing the image on a surface of a photoconductor with or without applying various image processes to the image.
Generally, in copying machines, for forming an image of an original document at a desired magnification ratio by enlargement or reduction, the driving speed of a photoconductor is kept constant and the driving speed of a scanner scanning the original document for reading the image of the original document is made inversely proportional to the magnification ratio. Copying machines for office use are generally provided with a scanner having a flat contact glass so that a book can be placed on the flat contact glass for copying. An original document is placed on the contact glass with a surface carrying an image for reading facing down, and the image is read by a one-dimensional image sensor by moving an exposure optical system, which includes a lamp, a mirror, and so forth and is provided beneath the contact glass, in a sub-scanning direction by a driving system, such as a wire pulley or the like, such that the surface carrying the image is exposed with light emitted from the lamp. The emitted light is passed through a slit and the light reflected by the surface carrying the image is received by the image sensor. Therefore, when an image is enlarged or reduced, the moving speed of the exposure optical system is changed according to the magnification ratio.
On the other hand, in a case of copying machines for large size original documents, such as A1 or A0 size documents and so forth, it is unrealistic to use a contact glass capable of receiving the whole portion of a large size original document from the view points of the size and cost of the machines. In addition, large size original documents are mostly sheets, such as drawings, and a demand for copying large size books is small. Therefore, generally, the copying machines for large size original documents include a fixed exposure optical system and are configured such that an original document is conveyed by conveying rollers so as to be read by the fixed exposure optical system. This configuration also has an advantage in that a long sheet original document can be copied. When enlarging or reducing an image in this configuration, the original document conveying speed is changed according to the magnification ratio.
In copying machines for large size documents in which an original document is conveyed as described above, when the copying speed is not fast, it rarely occurs that the original document is damaged or tom when being conveyed, and measures for protecting the original document from being damaged are not particularly required. However, demand for increasing the copying speed has risen in recent years, and besides, the magnification ratio for reducing an image has increased, for example, to 25% from 75% or 50%, which used to be a generally available magnification ratio, which consequently causes the original document conveying speed to be further increased.
Generally, in digital copying machines, enlargement and reduction of an image in the main scanning direction is accomplished by electrical processes of interpolation and omission of image information of one line of the image information which is read by a scanner and stored in a line memory. For the sub-scanning direction, however, because costs would be greatly increased if a frame memory including line memories for storing a number of lines of image information was used for accomplishing electrical enlargement or reduction, a so-called mechanical enlargement and reduction method in which the reading speed of a scanning device is made inversely proportional to the magnification ratio is widely used. When changing the driving speed of a scanner to one inversely proportional to the magnification ratio, for example according to Japanese patent laid-open publication No. 5-63905, the driving speed of a driving motor is slowly increased such that the driving speed of the scanner does not greatly overshoot a target driving speed.
In a case of copying machines for office use, even if the driving speed of a scanner is increased, because the driving system of the scanner is contained in a body of the machine and the original document is not conveyed, protection of the original document and measures for protecting an operator are not necessary. On the other hand, in a case of copying machines for large size documents, in which the original document is conveyed for reading, often a part of the original document conveying system is exposed. When the original document conveying speed is fast, there exists a danger that an original document of thin paper may be damaged or torn or a finger of an operator inserting the original document in the copying machine may be caught by the original document conveying system. Accordingly, it is not desirable to increase the original document conveying speed too greatly. For preventing an original document from being damaged, a known method limits the original conveying speed to a certain level by switching the enlargement and reduction process from the mechanical one to an electrical one when a desired magnification ratio for reduction exceeds a predetermined magnification ratio.
However, when the electrical enlargement and reduction method is used for the sub-scanning direction in a copying machine using a method of conveying an original document for reading, an operation of forming an image of a long original document synchronized with the movement of the long original document becomes impossible, which is possible with the mechanical enlargement and reduction method. More specifically, when the original document is long and when feeding of a transfer sheet at a printer is started at the same time that the original document is inserted into a scanner and image information of the original document read by the scanner is started to be stored in a frame memory, if the read image information overflows the capacity of the frame memory, the operation of forming an image is interrupted and terminated.
When the size of an original document is a standard size, such as A1 or A2, because the length of the original document is known, it is possible to start feeding of a transfer sheet at a printer after reading image information of the original document to an appropriate portion of the document and to store the read information in an image memory so that the image forming operation will not be terminated in the course of the image forming operation due to overflowing of the read image information from the image memory. However, in a case of a long original document and when the length is unknown, even when feeding of a transfer sheet is started after reading of the original document is completed up to the maximum capacity of the image memory, if the remaining image information of the original document exceeds the capacity of the memory, the image forming operation has to be terminated in the course of the image forming operation.
The present invention has been made in view of the above-discussed and other problems and addresses and resolves the above-described and other problems.
Preferred embodiments of the present invention provide a novel digital image forming apparatus and method which is capable of suppressing an original document conveying speed even when the magnification ratio for reduction is large and for forming an image of a long original document synchronized with conveying of the original document.
According to a preferred embodiment of the present invention, a novel image forming apparatus includes an original document conveying device to convey an original document at a predetermined conveying speed according to a magnification ratio. An original document reading device exposes the original document conveyed by the original document conveying device and reads image information of the original document with a one-dimensional image sensor, and an information omission device omits information of the image information of the original document read by the original document reading device. An image forming device forms an image according to the image information read by the original document reading device synchronized with conveying of the original document by the original document conveying device. An original document protect mode designating device designates an original document protect mode, and an original document protect mode processing device reduces the predetermined original document conveying speed of the original document conveying device by 1/n and causes the information omission device to omit information of every (nxe2x88x921) number of lines of the image information read by the original document reading device in a sub-scanning direction when the original document protect mode is designated by the original document protect mode designating device.
Accordingly, in the image forming apparatus of the present invention, when forming an image of a long original document reduced at a certain magnification ratio, for example, by designating the original protect mode with the original protect mode designating device, the original conveying speed of the original document reading device according to the magnification ratio is reduced, for example by half, and at the same time image information of the original document read by the original document reading device is omitted for every other line. As a result, an image reduced with the desired magnification ratio is formed without damaging the original document and synchronized with conveying of the original document.